Glass frit containing lead ruthenate or lead iridate in relatively uniform dispersion and method to produce same

ABSTRACT

Thick film resistive elements are prepared from an intimate admixture comprising silica, lead oxide and ruthenium dioxide or iridium dioxide which is heated to a temperature sufficient to provide a lead-containing glass having dispersed therein lead ruthenate or lead iridate. The lead-containing glass is comminuted and a resistor paste is formed which can be subsequently coated and fired on to a desired substrate to form the thick film resistive component. The method of this invention may facilitate the production of thick film resistors exhibiting a low temperature coefficient of resistivity, relative freedom from noise and drift, and high moisture resistance.

This is a continuation of application Ser. No. 301,102, filed Oct. 26,1972, now abandoned.

The present invention relates to the manufacture of thick film resistiveelements, and particularly to a method of making a resistive compositionwhich exhibits a low temperature coefficient of resistivity (TCR),relative freedom from noise and drift, and high moisture resistance. Theproperties of the resistive elements produced by the present inventionare relatively insensitive to minor variation in process conditionsduring their manufacture and are able to be reproduced with a relativelyhigh degree of accuracy, and separate elements exhibiting widelydiffering resistance can be made.

In accordance with the present invention glass frit suitable for themanufacture of thick film resistives is prepared from an intimateadmixture comprising silica, lead oxide and ruthenium dioxide or iridiumdioxide wherein the lead oxide-containing glass is produced and leadruthenate or lead iridate is formed in situ and is well dispersed assolid particles in the glass matrix. The lead ruthenate or lead iridatecomponent serves as an electrical conductor in a resistive element.Additionally, other materials, especially a boria component, may beincorporated into the glass thereby providing a modified glass, e.g. alead borosilicate glass. Thick-film resistors may be produced bycomminuting the lead oxide-containing glass, formulating the comminutedglass into a resistor paste composition, applying the resistor paste toa substrate, and baking the coated substrate. Other materials may alsobe included in the composition, for instance, otherelectrically-conductive metal components.

As electronic circuitry becomes more complex and emphasis is placed onnot only high performance, but also the miniaturization of suchcircuitry, new techniques for circuit fabrication and circuit design arerequired. Widespread adoption of printed circuits and hybrid integratedcircuits has resulted to achieve miniaturization of electroniccomponents. The development of thick film passive circuit elements isone factor which has enabled the manufacture of such microcircuits. Thecomponents for thick film elements which exhibit various electricalcharacteristics are in the form of fine powder which can be consolidatedon a solid substrate by firing. The powder is usually applied to thesubstrate in a paste form using a graphic arts process, and theresulting film is often in the neighborhood of about 0.2 to 1.5 or moremils in thickness. Thick film resistive elements are normally formedfrom a mixture of finely divided ceramic powder such as glass, ceramics,and glazed (glass coated) ceramics, with fine metal-containingparticles. These resistive elements are commonly referred to as "cermet"resistive elements since they are derived from a combination of ceramicand metal materials. The formation of the thick film should desirably bereproducible in order that a thick film resistive element can bemanufactured with a high degree of reliability and uniformity ofelectrical characteristics and without undue sensitivity to minorchanges in the process conditions during manufacture. Further, due totheir employment in complex circuitry, the thick film resistive elementsshould exhibit a relative freedom from undue variation in electricalcharacteristics due to changes in environmental conditions such astemperature, pressure, and humidity. Thick film resistors should be freefrom rough surface characteristics, high noise characteristics; unduedrift, and the like which impair beneficial employment in circuitry andshould exhibit a high moisture resistance.

The resistance of thick film resistive elements, referred to as sheetresistance, is usually measured in "ohms per square", a parameter whichconsiders only the amount of area taken up on a substrate by a givenresistance. Resistance values of thick film resistors are obtained bymeasuring the resistance between parallel sides of the film and dividingthat value by the least number of geometric squares formed on thesurface of the film having the width of the film as one side. Typically,a uniform film thickness of 1 mil is employed. Temperature coefficientof resistance (TCR), generally expressed in parts per million per degreecentigrade (ppm/° C), is an important characteristic of resistors sincechanges in temperature can create relatively large changes inresistance. A value for TCR is generally obtained by measuring theresistance of a resistive element at various temperatures, and often thevariation of resistance of a resistive element due to a change intemperature is non-linear in nature. If the TCR is too high, inevitablechanges in ambient temperature in many modern applications of electroniccircuits could lead to serious consequences. Thus, if a resistanceelement with a TCR of 1000 ppm/° C. were used in a circuit subjected toa 100° C. change in ambient temperature, the resistance thereof wouldchange by a factor of 10%.

Ruthenium dioxide and iridium dioxide have been employed in resistivecompositions in the prior art. These compositions have, under carefullycontrolled conditions, exhibited advantageous properties including lowTCR and relative freedom from drift and noise. Often, however, variousof the prior art processes require several process steps each of whichmay affect the properties of a resistor made therefrom, and theproperties of the resistors, while being improved from conventionalresistor materials, often still do not meet the exacting qualitiesdesirable for modern applications nor are they reproducible within areasonable degree of variation.

There is provided by the present invention a method for the productionof cermet compositions which are suitable for use as thick filmresistive elements having low TCR values. Thick film resistors made inaccordance with the present invention demonstrate, in addition toimproved TCR, other desirable characteristics such as low noise, freedomfrom undue rough surfaces, improved moisture resistance, and low driftafter extended use. Where the TCR of a resistor element is relativelyunimportant, for instance, where the electronic component is operatingat a constant temperature in, for example, a sensitive infrareddetecting apparatus where the performance of the apparatus is enhancedby maintaining it at low, constant temperatures, often at thetemperature of liquid nitrogen, the resistive elements of the presentinvention are still highly advantageous not only due to good electricalproperties but also to the uniformity and ease of manufacture of theresistive material. The process of the present invention facilitates theproduction of thick film resistors having a high degree ofreproducibility and uniformity with minimal sensitivity to changes inthe process conditions employed.

By the present invention, glass frit, suitable for incorporation intothick film resistors, can be prepared by admixing the ingredients inamounts used to form conventional lead oxide-containing glasses, e.g.,silica and lead oxide components, with a minor amount of one or both ofruthenium dioxide and iridium dioxide. The mixture is then heated to atemperature which will promote the formation of lead ruthenate or leadiridate, generally at least about 600° C., often about 850° to 1100° C.,preferably about 900° to 1050° C. for a time sufficient to form a glasscontaining lead ruthenate or lead iridate, usually for at least about0.3 or 0.5 of an hour. Times exceeding about 24 hours do not appearnecessary, especially when higher temperatures are employed, andpreferably the time is about 0.5 to 16 hours. Preferably, thetemperature and time of the glass-forming heating is sufficient toconvert a major portion, more preferably essentially all, of theruthenium dioxide or iridium dioxide to lead ruthenate or lead iridate.The formed glass is cooled and comminuted or micronized, usually afterquenching, to provide a frit of a suitable particle size which canadvantageously be incorporated into a resistor paste composition for themanufacture of thick film resistors. The glass frit of this inventionnot only has highly advantageous electrical properties, but also otherphysical properties which facilitate its employment in thick filmresistors since the lead ruthenate or lead iridate-containing glass fritbehaves essentially the same as single phase glass.

The relative amounts of silica, lead oxide and ruthenium dioxide oriridium dioxide components used in making the glass of this inventionmay vary over a wide range. The lead ruthenate and lead iridate presentin the formed glass is usually a minor portion based on the weight ofthe glass, but the amount is sufficient to provide anelectrically-conductive glass. Advantageously, the total mole ratio oflead oxide and silica to the total ruthenium dioxide or iridium dioxideused in making the glass is about 2:1 to 50:1, preferably about 5:1 to40:1. The ratio of lead oxide to silica is normally about 0.l:1 to 4:1,preferably about 1:1 to 2:1, on a mole basis. The lead oxide and silicacombine during formation of the glass to provide lead silicate.Preferably the ruthenium dioxide and iridium dioxide are finely dividedand, most preferably, are less than about 150 or 50 microns in averageparticle size. The preferred essential conductive component of the glassfrit is lead ruthenate formed from ruthenium dioxide or lead iridateformed from iridium dioxide.

Other oxide components may be incorporated into the glass compositionemployed in this invention, such as, for instance, oxides of calcium,strontium, aluminum, cadmium, boron and the like. A particularlyadvantageous component is boria which combines with lead oxide andsilica to form lead borosilicate. Lead borosilicate glasses possessgenerally lower melting points than do lead silicate glasses and oftenprovide ruthenium- or iridium-containing glass frits having a morestable TCR. Advantageous lead borosilicate glasses have essentiallyidentical thermal coefficients of expansion to common substratematerials, for example, alumina. Desirably, the ratio of lead silicateto boria is from about 3.5:1 to 30:1, preferably about 4:1 to 25:1, on amole basis. Opacifiers and other agents which improve the physicalproperties of the glass may also be employed. Since ruthenium is in thepreferred conductive component, the remaining discussion will primarilybe directed to the use of a ruthenium component; however, this is forease of understanding the present invention and it is to be understoodthat the ruthenium component may be replaced entirely or only in part bythe corresponding iridium component.

The mixture of the glass-forming components and ruthenium dioxide oriridium dioxide is heated to a temperature sufficient to form a moltenmass of glass. Ruthenium dioxide or iridium dioxide, being relativelyinsoluble in glass will remain in the solid phase. Glasses, in general,have relatively imprecise softening points and gradually melt, formingless viscous solutions as the temperature is increased. Advantageously,the glass-forming components employed in the present invention will beheated in their molten state to a temperature at which the viscosity issufficiently high in order to retain the solid ruthenium or iridiumcomponents in dispersion. Preferably, the solid ruthenium- oriridium-containing particles are in relatively uniform dispersion withthe glass matrix. The temperature which will provide a solution ofadvantageous viscosity may vary with the composition of the glass. Forinstance, lead borosilicate glasses generally have a lower melting pointthan lead silicate glasses. Similarly the relative amounts of each ofthe components will have an effect upon the melting point of the glass.Generally the melting point of the glass composition in this inventionwill be from about 550° to 800° C.

Along with the formation of lead silicate glass, the ruthenium dioxidewill react with the lead component in the glass composition to form leadruthenate, The reaction proceeds in the solid phase. The product, leadruthenate, is relatively insoluble in the glass and is present in thesolid phase. Usually at least a major amount, preferably essentiallyall, of the ruthenium is converted to the bimetal salt. When employingtemperatures in excess of 1000° C., it may be desirable to conduct theheating in an inert atmosphere. Free oxygen from, for example, air mayreact with ruthenium dioxide or lead ruthenate to form rutheniumtetroxide which will be a gas at those conditions. High temperatures,e.g., above about 1100° C., may promote the degradation of leadruthenate. The time for which the solution is maintained at thetemperature for conversion of ruthenium dioxide to lead ruthenate maydepend upon numerous considerations such as the temperature employed,the relative concentration of ruthenium, the relative concentration oflead, the desired degree of conversion, and the like. Generally, thesolution will be maintained at the peak temperature for about 0.3 to 24,preferably 0.5 to 16, hours. It may be desirable to provide a means foragitating the mixture to maintain the insoluble lead ruthenate in arelatively uniform dispersion. Surprisingly, by maintaining a suitableviscosity for the glass melt, the lead ruthenate can be kept in areasonably uniform dispersion without agitation.

The molten glass is solidified by cooling. Desirably, the molten glassis immediately quenched in cool or cold water. The quenching serves atwo-fold purpose. First, the quenching will result in the shattering ofglass, thus the amount of grinding required to obtain frit of thedesired particle size is reduced, and second, the quenching serves tofix the relatively uniformly dispersed lead ruthenate in the glassmatrix. The glass is preferably comminuted or micronized to an averageparticle size of less than about 20 microns, most preferably less thanabout 10 microns, e.g., about 0.5 to 5 microns. The comminuting of theglass may be accomplished by, for example, ball milling and the like.Methanol, ethanol, water, and the like may be conveniently employed as aliquid phase material for ball milling. The comminuted glass frit may bestored indefinitely without significant, if any, deterioration.

The comminuted glass frit containing the lead ruthenate can be preparedinto a resistor paste for use in forming thick film resistors. The termresistor paste as used herein refers to pastes or more fluid slurrycompositions. The glass frit may be admixed with up to about 90,preferably about 5 or 10 to about 80, weight percent additional glassfrit to adjust the metal component concentrations. The additional glassfrit may be used as such or have a metal component therein. Thus, forinstance, by increasing the amount of added glass frit, the sheetresistance of a resulting thick film resistor is increased. A metalcomponent such as about 1 to 10 weight percent precious metal, forexample, platinum, rhodium, and the like based on the additional glassfrit may be useful for thick film resistors. Such metal components mayalso be incorporated in or on the lead ruthenate or leadiridate-containing glass. Advantageously, the resistor paste can beprepared from an admixture of glass frit of low resistance, e.g., about1 to 100 ohms per square, and glass frit of high resistance, e.g., about50K to 100K ohms per square, made in accordance with this invention. Theratio of each glass frit can be varied to obtain the desiredresistivity. The glass frit components can be incorporated into a pasteby mixing or milling the glass frit with a liquid vehicle, which mayinclude a thickener, e.g., ethyl cellulose, rosin or the like; a liquidcarrier such as methanol, ethanol, acetone, methyl ethyl ketone,terpineol, pine oil, oil of spike, camphor oil, lavender oil, oil ofpetitgrain, other organic solvents, water and the like; and, optionally,stabilizing agents and wetting agents. The resulting resistor paste mayoften have about 50 to 80 percent solids and about 20 to 50% vehicle.The viscosity of the resistive paste may affect the thickness of thethick film resistor and, hence, may affect the resistance of theresistor thusly formed. The resistor paste may be applied to a suitablebase or substrate by various convenient means such as brushing,spraying, stenciling, screening, printing and the like. Beneficially,the method of application of the resistor material provides a thick filmcoating of relatively uniform thickness. Typical solid substratematerials are electrically non-conductive, able to withstand the hightemperatures used in firing the resistor to the substrate, have asmooth, fine textured surface characteristic, and are virtuallyimpervious to moisture and other liquids. Often, the substrate is of aceramic nature. Steatite, fosterite, sintered or fused aluminas, zirconporcelains, and the like, can be employed as substrates.

A further consideration in the production of thick film resistors istheir adaptability to other circuit elements, such as conductors. Thethick film resistor should bond securely and without imperfection whichwould adversely affect its electrical characteristics. For instance, theblistering or separation of the resistive material from a conductorwould provide an increase in the resistance of the unit and nosatisfactory reliability or predictability in manufacture could beachieved. The resistive compositions of the present invention have beenfound to be particularly compatible with platinum conductive elements.The conductor component which may be bonded to thick film resistors isalso desirably selected to be compatible with the resistor with respectto physical properties such as thermal expansion, especially where thecircuit is subjected to widely varying temperatures. It has been foundthat a conductor comprising a glass frit having similar physicalproperties, e.g., thermal expansion, melting characteristic, and thelike, to the resistor component and a finely-divided,electrically-conductive component is advantageous. Theelectrically-conductive component is in a sufficient amount to provide aconductor with the desired electrical properties and may comprise about50 to 99, preferably 80 to 98, percent of the conductor. Excellentbonding of the thick film resistor of this invention to the conductorhas been observed using platinum as the electrically-conductivecomponent in the conductor. The glass frit for the conductor mayconveniently be the same glass frit as used for the thick film resistor.

After the resistor paste is applied to the substrate, it is normallyallowed to dry by evaporating the carrier at a low heat. Warm air may becirculated over the applied resistor paste to assist in evaporation ofthe carrier. The vehicle employed in the resistor paste will generallycontain sufficient binder that, when dried, the surface of the driedresistive paste will be sufficiently strong such that the substrate canwithstand normal handling without marring or blemishing the driedresistive paste.

The resistive material can then be fired to fuse the frit into acontinuous glassy phase in a conventional lehr or furnace by graduallyincreasing the temperature to a peak temperature of at least about thetemperature at which the frit becomes molten and a smooth, continuousglass phase is formed. Firing temperatures which are too low may providea discontinuous resistive element providing erratic resistance values.Desirable temperatures for firing are in the range of about 600° toabout 900° C., preferably about 600° to 850° C. The conditions employedfor fusing the resistor paste to the substrate may be less severe thanthose employed for the formation of the lead ruthenate, e.g., generallya temperature of at least about 100° C. less than that for the formationof the lead ruthenate is employed, since the lead ruthenate isessentially completely formed in the melt heating step. The furnace ispreferably held at the maximum peak firing temperature for at leastabout five minutes to insure the production of a continuous glassy phasewith a smooth surface. Often the substrate is held or soaked at themaximum peak firing temperature from about 10 to 30 minutes or more.Excessive peak temperatures and fast heating rates may cause blisters orbubbles on the thick film resistor and may cause agglomeration of themetallic components. Substrates such as alumina may react with the glassfrit containing lead ruthenate to an undesirable extent at hightemperatures, e.g., in excess of about 1000° C. The temperature of thefurnace can be slowly reduced after reaching and maintaining the desiredpeak temperature to insure that the thick film resistor is relativelyfree from spalling or undue stresses due to more rapid cooling which mayaffect the performance or properties of the resistor.

The lead ruthenate- or lead iridate-containing glass frit is anadvantageously employed thick film resistor due to the beneficial fusiontemperatures, coefficient of thermal expansion, fluidity, and the like.The thick film resistors are moisture resistant, fuse to a smooth glossysurface upon heating to a temperature above the melting point of theglass, and have low TCR, relative freedom from noise and drift, andrelative insensitivity from minor variations in the process condition oftheir manufacture. The process of this invention enables the productionof thick film resistors with uniform and predictable electricalcharacteristics and provides for the economic and efficient use of metalvalues. The thick film resistors prepared with the glass frit of thisinvention can provide a range of elements of low or high resistivitywith a low TCR. The higher resistances may be in excess of about 5,000or 8,000 to 1,000,000 or more ohms per square, and products can be madewhich show little, if any, change in electrical characteristics uponextended use.

The following examples are presented to further illustrate the presentinvention. All parts and percentages referred to are by weight unlessotherwise indicated. In the following examples, the resistive element isprepared essentially in the same manner as illustrated in Example I.

EXAMPLE I

An admixture of 22.32 grams (g.) lead oxide, 4.50 g. silica, and 3.33 g.ruthenium dioxide (mole ratio of PbO:SiO₂ :RuO₂ being 4:3:1) is preparedhaving essentially uniform dispersion. This mixture is placed in aporcelain crucible and heated to 1000° C. for 30 minutes to provide amolten solution having a dispersion of ruthenium-containing components.This material is quenched in water and ground and sieved through a 325mesh screen (U.S. Standard Sieve Series) to give a powder. X-raydiffraction of this material in powder form shows that it is a mixtureof two crystalline phases, lead ruthenate and ruthenium dioxide, in anamorphous phase. The resultant glass frit is formulated in a resistivepaste by admixing 7 parts by weight of the powder with 2 parts rosin(50%) dissolved in oil of spike, 1/3 part lavender oil, 1/3 part camphoroil, and 1/3 part oil of petitgrain. The paste is screened onto analumina substrate to a thickness of about 0.6 to 1.2 mils. The film isdried and then fired at 800° C. for 30 minutes. The thick film resistiveis a continuous film of a smooth, glassy appearance and is bonded wellto the substrate. Approximate resistivity measurements are made with thefilm exhibiting a resistance of 90 ohm-cm at 25° C., 100 ohm-cm at 100°C., and 110 ohm-cm at 200° C.

EXAMPLE II-VI

Example I is repeated except that varying amounts of glass-formingcomponents to ruthenium dioxide are used. The results are provided inTable 1. Examples IV and V are for comparison purposes. The erraticresistivity readings are probably due to the low concentration ofelectrically-conductive material.

                  Table 1                                                         ______________________________________                                        Moles of             Resistivity at 25° C.,                            Example                                                                              PbO      SiO.sub.2                                                                              RuO.sub.2                                                                           ohms--cm                                       ______________________________________                                        II      8        6       1     6K                                             III    10       12       1     120K                                           IV     32       24       1     erratic                                         V     64       48       1     erratic                                        VI      2        1       1     10                                             ______________________________________                                    

EXAMPLE VII

Example I is repeated except that the ruthenium dioxide is ground topass through a 325 mesh screen. The thick film resistor has aresistivity of about 12 K ohm-cm at about 100° C.

EXAMPLES VIII to X

Example VII is repeated except the meting is conducted at 1000° C. for16 hours and the thick film resistors are fired at varying temperatures.The results are provided in Table 2.

                  Table 2                                                         ______________________________________                                                Peak firing temperature,                                                                       Resistivity,                                                                             TCR                                       Example °C (30 minutes)                                                                         ohms       ppm/°C                             ______________________________________                                        VIII    700              70K        -400                                      IX      800              4K         +230                                       X      900              8K         +330                                      ______________________________________                                    

The samples prepared at 700° C. are dull and granular in appearance.This is apparently due to the use of an inadequate temperature toprovide melting of the glass frit.

EXAMPLE XI

Example IX is repeated except employing a composition having a moleratio of lead oxide to silica to ruthenium dioxide of 8:7:1. Theresulting resistor has a resistivity of 50 K ohms and a TCR of +80 ppm/°C. Further samples of the resistor of this example are prepared with avariation in TCR from +358 to -630 ppm/°C. This variation is attributedto a thermal expansion mismatch with the substrate and due todifferences in the resistor film thickness due to simple handapplication of the resistor paste to the substrate.

EXAMPLE XII

A mixture of 1 part by weight lead borate per three parts of thecomposition of Example XI is prepared and treated in the same manner asin Example XI. The resistor paste is applied to alumina substrates andfired at 700° C., 750° C., and 800° C. for 30 minutes. The results areprovided in Table 3.

                                      Table 3                                     __________________________________________________________________________    Peak                       Average TCR, ppm/°C                         firing No. of                                                                             Highest TCR                                                                           Lowest TCR                                                                           dropping highest and                               temperature                                                                          Samples                                                                            ppm/°C                                                                         ppm/°C                                                                        lowest value                                       __________________________________________________________________________    700°C                                                                         7    +196    +19    +69                                                750°C                                                                         1    +102     0     +34                                                800°C                                                                         14   +312    +66    +170                                               __________________________________________________________________________

EXAMPLE XIII

This example illustrates the effect of a melt which is heated to atemperature which for this formulation provides a melt having aviscosity which is insufficient to maintain a satisfactory dispersion oflead ruthenate. A mixture of 17.83 g. lead oxide, 3.60 g. silica, 7.70g. lead borate, and 1.33 g. ruthenium dioxide which passes through a 325mesh screen is prepared and heated in a platinum crucible to 1000° C.for 16 hours. The mixture is noticeably relatively fluid. The melt ispoured into water and is ground to pass through a 325 mesh screen.Resistors are prepared in the usual manner and the results are providedin Table 4.

                                      Table 4                                     __________________________________________________________________________    Peak                       Average TCR, ppm/°C                         firing No. of                                                                             Highest TCR                                                                           Lowest TCR                                                                           dropping highest and                               temperature                                                                          Samples                                                                            ppm/°C                                                                         ppm/°C                                                                        lowest value                                       __________________________________________________________________________    650°C                                                                         4    710     411    549                                                700°C                                                                         4    478     439    467                                                750°C                                                                         4    716     433    455                                                __________________________________________________________________________

The high TCR readings are apparently caused by the inability of the leadruthenate to remain in uniform dispersion in the relatively fluid melt.

EXAMPLE XIV

Example XIII is repeated except only 3.10 g. of lead borate are used.The heated mixture is more viscous at the maximum melt temperature, andlower TCR values are obtained. The results are presented in Table 5.

                                      Table 5                                     __________________________________________________________________________    Peak                       Average TCR, ppm/°C                         firing No. of                                                                             Highest TCR                                                                           Lowest TCR                                                                           dropping highest and                               temperature                                                                          Samples                                                                            ppm/°C                                                                         ppm°C                                                                         lowest value                                       __________________________________________________________________________    650°C                                                                         only partial melting occurred                                          700°C                                                                         6    -281    +42    -119                                               750°C                                                                         7    +242     +4    +148                                               __________________________________________________________________________

EXAMPLE XV

Example XIV is repeated except employing 1.55 g. lead boratemonohydrate. The peak firing temperatures are 700°, 750°, and 800° C.for 30 minutes. The resistors fired at 700° C. only partially melted andvalues for TCR are not obtained. The results are provided in Table 6.

                                      Table 6                                     __________________________________________________________________________    Peak                       Average TCR, ppm/°C                         firing No. of                                                                             Highest TCR                                                                           Lowest TCR                                                                           dropping highest and                               temperature                                                                          Samples                                                                            ppm/°C                                                                         ppm/°C                                                                        lowest value                                       __________________________________________________________________________    750°C                                                                         6    -320     -8     -40                                               800°C                                                                         6     306    138    +198                                               __________________________________________________________________________

EXAMPLES XVI and XVII

Example XV is repeated except no preforming of the glass is employed,i.e., the glass is formed on the substrate. In Example XVII, 0.75 micronparticle size ruthenium dioxide powder is employed, but the preparationof the sample is otherwise the same as Example XVI. The results areprovided in Table 7.

                  Table 7                                                         ______________________________________                                                TCR, ppm/°C.                                                   Example   Run    1           2       3                                        ______________________________________                                         XVI             0          36       233                                      XVII             271        327      1234                                     ______________________________________                                    

Without the preforming step, the values obtained for TCR apparently varyconsiderably more.

EXAMPLE XVIII

Example XV is repeated except that the melt is heated to a maximumtemperature of 900° C. The melt at 900° C. pours easily into water.X-ray diffraction indicates that lead ruthenate is the only crystallinephase. Thus, essentially all of the ruthenium dioxide is converted tolead ruthenate under the conditions employed in forming the melt.Further, the electrical measurements indicate that the viscosity of themelt at the peak temperature of 900° C. is advantageous in maintainingthe lead ruthenate particles in a substantially uniform dispersion. Theresistive material is employed to make resistors of short (about 1/16inch), average (about 3/16 inch), and long (about 1/2 inch) lengths andthe resistors are defined at different temperatures. The results areprovided in Table 8.

                                      Table 8                                     __________________________________________________________________________    Peak                    Average TCR, ppm/°C                            firing    Time,    No. of                                                                             dropping highest and                                  temperature, °C                                                                  min.                                                                              Length                                                                             Samples                                                                            lowest value                                          __________________________________________________________________________    700       30  average                                                                            7    -170                                                  750       30  long 10    51                                                   750       60  average                                                                            11    82                                                   750       30  short                                                                              8    -40                                                   800       30  average                                                                            9    165                                                   __________________________________________________________________________

EXAMPLE XIX

Example XVIII is repeated except employing iridium dioxide instead ofruthenium dioxide in an equivalent amount on a mole basis. A mixture of17.84 g. lead oxide, 3.60 g. silica, 1.52 g. lead borate monohydrate and2.25 g. iridium dioxide is prepared and heated to 900° C. for 16 hours.It is formulated in a resistor paste in the usual manner and printed onan alumina substrate with platinum terminals. The resistor is fired at750° C. for 30 minutes. The number of runs conducted are seven with thehighest TCR being 503 ppm/° C. and the lowest, 139 ppm/° C. The averageTCR is 322 ppm/° C.

EXAMPLE XX

Example XVIII is repeated except that the conductor material from thethick film resistor is derived from 95 parts platinum and 5 parts of theresistive composition of Example XVIII. This conductor enables a simplesystem to be provided for the manufacture of circuits containing thickfilm resistors which have increased stability due to the use ofidentical glass phases in the resistor and the conductor element. Theresistors are fired at 750° C. for 30, 60, and 90 minutes and at 800° C.for 10, 20, and 30 minutes. The results are provided in Table 9.

                  Table 9                                                         ______________________________________                                                                      Average TCR°C                            Peak firing Time,    No. of   dropping highest and                            temperature, °C                                                                    minutes  Samples  lowest value                                    ______________________________________                                        750         30       11       120                                             750         60       11       -21                                             750         90       18        20                                             800         10       10       157                                             800         20       10       130                                             800         30       11       177                                             ______________________________________                                    

It is claimed:
 1. A method of making a glass frit suitable for use inthe production of thick film resistors comprising heating afinely-divided mixture of glass-forming proportions of lead oxide andsilica, and a minor amount of a member selected from the groupconsisting of ruthenium dioxide and iridium dioxide to a temperaturesufficient to form a glass melt containing lead ruthenate or leadiridate, whereat the viscosity of the melt is sufficiently high tomaintain the lead ruthenate or lead iridate in relatively uniformdispersion in the glass melt, cooling and comminuting the mixture toprovide the glass frit.
 2. The method of claim 1 wherein the temperatureof the melt is from about 850° to 1100° C.
 3. The method of claim 2wherein the temperature of the melt is from about 900° to 1050° C. 4.The method of claim 2 wherein the said temperature of the melt ismaintained for about 0.3 to 24 hours.
 5. The method of claim 1 whereinruthenium dioxide is employed as the selected member.
 6. The method ofclaim 1 wherein the melt is cooled and comminuted to an average particlesize of less than about 20 microns.
 7. The method of claim 6 wherein theselected member is less than about 150 microns in average particle size.8. The method of claim 1 wherein the mole ratio of lead oxide to silicais from about 0.1:1 to 4:1 and the mole ratio of total lead oxide andsilica to total ruthenium dioxide and iridium dioxide is from about 2:1to 50:1.
 9. The method of claim 1 wherein boria is provided in themixture and the product contains lead borosilicate glass.
 10. The methodof claim 9 wherein the mole ratio of total lead oxide and silica toboria is from about 3.5:1 to 30:1.
 11. The method of claim 2 wherein themole ratio of lead oxide to silica is from about 0.1:1 to 4:1 and themole ratio of total lead oxide and silica to total ruthenium dioxide andiridium dioxide is from about 2:1 to 50:1.
 12. The method of claim 11wherein ruthenium dioxide is employed as the selected member.
 13. Themethod of claim 12 wherein the mole ratio of total lead oxide and silicato boria is from about 3.5:1 to 30:1.
 14. A product of the method ofclaim
 1. 15. A product of the method of claim
 5. 16. A product of themethod of claim
 6. 17. A product of the method of claim
 7. 18. A productof the method of claim
 8. 19. A product of the method of claim
 9. 20. Aproduct of the method of claim 10.